_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Alkali-metal derivatives
Table 3. 3. Acidity values (pKa) of hydrocarbons
pKa pKa pK"
Me,CH 47 PhH 39 Ph,CH 32
Me 2 CH 2 44 H,C=CH, 38.5 HC=CH 24
MeCH, 42 PhCH, 37 Cyclo- 18.5
pentadiene
CH 4 40 Ph,CH, 33.5 Fluorene l 5
The reaction goes to the right if the hydrocarbon R'H is more acidic than RH ( or
R 2 NH). In Table 3.3 the pK" values of some hydrocarbons, a measure of their
acidities, are listed. Phenylethyne. for example, reacts with either butyl- or
phenyllithium
PhC-CH + PhLi ---> PhC=CLi + PhH
Metallation is believed to involve nucleophilic attack by the organolithium
reagent on the 'acidic' hydrogen atom. The nucleophilic character of carbon
bonded to lithium is enhanced by coordination of the lithium to a base.
Organolithium compounds are therefore more reactive when dissolved in ethers
than in hydrocarbons. Unfortunately ethers can themselves be metallated. This is
especially a problem with the rather basic ether tetrahydrofuran, which is rapidly
cleaved by butyllithium at room temperature.
O
-BuH
+ Buli
o
-
This difficulty does not arise with tertiary amines since the C-H bonds adjacent
to nitrogen are less susceptible to nucleophilic attack than those next to oxygen
in ethers. TMEDA (p. 38) complexes strongly with alkyllithiums. The n­
butyllithium chelate is monomeric and very soluble in hydrocarbons. It is a very
strong metallating agent. It reacts with toluene at room temperature and more
slowly, with benzene. Butyllithium in the absence of TMEDA does not normally
attack benzene or toluene and can even be prepared in these solvents.
Aroma tic compounds which bear electron attracting (-1) substituents are
susceptible to metallation by butyllithium. On account of the inductive effect the
acidity of the ortho hydrogen atoms in particular is enhanced. Methoxybenzene,
for example, affords predominantly the ortho-lithium compound. In some cases
the ortho-directing effect is increased through coordination of the substituent to
lithium. The reagent is thus held near to the site of attack, (Fig. 3.6). In spite
of this, metal-hydrogen exchange is less regiospecific in general than
metal-halogen exchange. As mentioned above alkylbenzenes (with + I substituents)
are metallated by butyllithium only in the presence of a strong base. Sodium
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